Monoclonal antibodies have been generated that are reactive with human colon carcinomas. The rationale for the studies was to utilize extracts from patient biopsy material (and not colon cancer cell lines) as immunogen to increase the probability that any monoclonal antibody generated be reactive with colon carcinomas in a clinical setting. Five immunization protocols were used employing extracts and membrane enriched fractions from both primary and metastatic colon carcinoma lesions. Seventeen monoclonal antibodies from double-cloned hybridoma cultures have been characterized; all are of the IgG isotope. Preliminary results indicate that the monoclonal antibodies can be placed into at least 6 groups on the basis of their differential reactivies to six colon carcinoma extracts, the surface of three colon carcinoma cell lines and five partially purified CEA preparations from bloods of colon cancer patients. Some of the monoclonal antibodies were shown to bind from one to all of the five CEA preparations tested, while others showed no anti-CEA reactivity. None of the monoclonal antibodies selected for further study reacted with extracts of 21 normal tissues including livers, spleens, kidneys, red blood cells, (of several blood groups), or polymorphonuclear leukocytes. All the monoclonal antibodies could be distinguished from antibodies previously generated in our laboratory. Further immunohistochemical and radiolocalization studies will further define the potential clinical utility of the monoclonals described.